Recently polymer-coated polyester base fabric has been employed to form very flexible unitary membranes which are custom fitted to various shapes, sizes and forms of roofs, particularly to what may be termed minimum pitched, roofs. Such roof coverings provide outstanding serviceability and weathering, accommodating at least light traffic when installed over existing roofs. Since the membrane material is very light in weight, weighing in the neighborhood of only 3.33 ounces per square foot, such membranes can be installed over old roofing, where structural weight limits are a major consideration.
Other features of such an all one-piece roof are the elimination of seepage, the excellent dimensional stability under humidity and temperature changes, and the added attributes that the roof will not absorb moisture, will not become stiff, inflexible or crack, and the further factor that the membrane resists the degrading effects of ultra violet rays, abrasions, and microorganisms, and has excellent resistance to many common chemicals.
One of the problems concerned with the installation of such one-piece, watertight membranes is what to do about existing metal vent pipes or stacks which must project through the roof covering. Formerly, when a vent pipe was to protrude, a separate tube was first formed of the membrane material, and a circular hole was cut in the membrane which was slightly smaller than the base of the vent pipe, thus creating a snug fit between the vent pipe and the material. The tube, which was slightly larger in diameter than the vent pipe, was then brought down over the vent pipe and pressed to the base against the flange of the roof pipe, after which the base of the tube was simply welded directly to the membrane material. Such vent tube enclosures often did not have a professional appearance, and were so relatively weak along the heat-welded seam that they could be torn apart by hand.